1. Field of the Invention
The present invention relates to an optical pickup for performing recording and/or playback of an information signal as to different three types of optical discs, and an optical disc device employing the same.
2. Description of the Related Art
In recent years, an optical disc capable of high-density recording (hereafter, referred to as “high-density recording optical disc”) wherein recording/playback of a signal is performed by employing an optical beam with a wavelength of 405 nm or so by a blue-violet semiconductor laser has been proposed as a next generation optical disc format. An example of this high-density recording optical disc has been proposed, which has a configuration wherein the thickness of a protection layer (cover layer) for protecting a signal recording layer is made thinner, for example, to 0.1 mm.
When providing an optical pickup compatible with these high-density recording discs, an optical disc having compatibility with optical discs having a different format such as a CD (Compact Disc) with a use wavelength of 785 nm or so, DVD (Digital Versatile Disc) with a use wavelength of 655 nm or so, and so forth, according to the related art has been desired. Thus, there has been demand for an optical pickup and optical disc device having compatibility with optical discs having formats wherein the disc configuration and accordingly laser specifications thereof differ.
Incidentally, each of optical discs such as the above-mentioned high-density recording optical disc, DVD, CD, and so forth has a standard (format) for protecting the uniformity and compatibility of pervasive media and devices for recording/playing these. With these written standards (format books), various requirements are described regarding product technology, but in order to realize so-called three-wavelength compatibility for realizing recording or playback of an information signal as to different three types of optical discs such as described above, it is particularly important to satisfy requirements regarding servo signals for focus servo or the like, which is one of the described items thereof.
In the case of performing such three-wavelength compatibility, it is desirable to realize a system compatible with high-density recording optical discs in addition to an existing system compatible with optical discs such as DVD, CD, and so forth, and accordingly, it is desirable to satisfy the requirement specifications of a focus error signal suitable for focus servo compatible with multiple types of formats. According to the difference between optical disc specifications determined with such multiple types of formats, the requirement specifications of a focus error signal suitable for focus servo compatible with each optical disc format differ.
In particular, the waveform of a focus error signal has close ties to the optical system and electric system of an optical pickup, an indication such as a sigmoid pull-in range (hereafter, also referred to as “focus capture range”) with the magnification of the return system of the optical system (hereafter, also referred to as “return magnification”) is restricted. Subsequently, in a case wherein this focus capture range is not adapted to each optical disc format, there has been a possibility to cause a problem wherein the sensitivity of focus servo deteriorates, defocusing occurs to prevent suitable focus servo, and thus, recording/playback is prevented from being performed in an appropriate manner. In order to provide a suitable focus capture range with each of the three wavelengths as use wavelengths, it is desirable to set the optimal return magnification as to each. The return magnification is determined principally with the focal length of the objective lens, the incident magnification as to the objective lens, and the focal length of an optical component disposed in other return optical systems. In the light of the above description, heretofore, as shown in FIG. 12, there has been employed or studied a method for providing two or more objective lenses of which the focal lengths differ to provide a predetermined return magnification difference between the respective wavelengths, and as shown in FIG. 13, a method for individually disposing multi lenses serving as a coupling lens of which the curvatures differ on an optical path other than a common path of the return system by employing two or more light receiving elements, and so forth. Now, description will be made regarding the configurations of optical pickups shown in FIGS. 12 and 13 which have heretofore been studied.
An optical pickup 130 shown in FIG. 12 realizes recording and/or playback of different types of optical discs by providing two types of objective lenses 134A and 134B, and includes a light source unit 132 such as a laser diode or the like including an emission unit for emitting an optical beam with a wavelength of 785 nm or so as to an optical disc such as CD or the like, and an emission unit for emitting an optical beam with a wavelength of 655 nm or so as to an optical disc such as DVD or the like, a light source unit 131 such as a laser diode or the like including an emission unit for emitting an optical beam with a wavelength of 405 nm as to a high-density recording optical disc, an objective lens 134B for optical disc such as DVD, CD, or the like, and an objective lens 134A for high-density recording optical disc. Also, this optical pickup 130 includes collimator lenses 137A and 137B, ¼ wavelength plates 145A and 145B, redirecting mirrors 146A and 146B, beam splitters 138 and 139, gratings 143 and 144, detector 141, multi lens 142, and so forth.
An optical beam with a wavelength of 785 nm or so emitted from the light source unit 132 transmits the beam splitters 138 and 139, and is input to the objective lens 134B. The optical beam is condensed on a signal recording face of an optical disc having a protection layer (cover layer) with thickness of 1.1 mm by the objective lens 134B.
Similarly, an optical beam with a wavelength of 655 nm or so emitted from the light source unit 132 is input to the objective lens 134B through the same optical path, and is condensed on a signal recording face of an optical disc having a protection layer with thickness of 0.6 mm. The return light with a wavelength of 785 nm and the return light with a wavelength of 655 nm which are reflected at the signal recording faces of the optical discs are detected by the detector 141 including a photodetector or the like through the beam splitter 139.
An optical beam with a wavelength of 405 nm or so emitted from the light source unit 131 is reflected at the beam splitter 138, and is input to the objective lens 134A through the beam splitter 139. The optical beam is condensed on a signal recording face of an optical disc having a protection layer with thickness of 0.1 mm by the objective lens 134A. The return light with a wavelength of 405 nm reflected at the signal recording face of the optical disc is detected by the detector 141 through the beam splitter 139.
With the return system of the optical pickup 130, the focal lengths and so forth of the objective lens 134B for DVD/CD and the objective lens 134A for high-density recording optical disc are adjusted as appropriate, whereby the return magnifications of the optical beams with a wavelength of 785 nm and wavelength of 655 nm for optical disc such as DVD, CD, and so forth, and the return magnification of the optical beam with a wavelength 405 nm for high-density recording optical disc can be set, and a focus capture range suitable for each of the wavelengths can be provided.
With the optical pickup 130 such as shown in FIG. 12, the two types of objective lenses of the above-mentioned objective lens 134B for DVD/CD, and the objective lens 134A for high-density recording optical disc are provided, thereby realizing recording and/or playback of different three types of optical discs, and also setting a focus capture range suitable for each of the optical discs, i.e., realizing compatibility between multiple types of optical discs.
However, with the optical pickup 130 shown in FIG. 12, the number of components increases, and the two objective lenses 134A and 134B are implemented in an actuator for driving objective lenses, thereby causing a problem such as increase in the weight of the actuator, deterioration in the sensitivity, and so forth. On the other hand, in order to eliminate such a problem, there has been studied an optical pickup 160 shown in FIG. 13 having a common objective lens compatible with three-wavelength optical beams.
The optical pickup 160 shown in FIG. 13 includes a light source unit 163 such as a laser diode or the like including an emission unit for emitting an optical beam with a wavelength of 785 nm or so as to an optical disc such as CD or the like, a light source unit 162 such as a laser diode or the like including an emission unit for emitting an optical beam with a wavelength of 655 nm or so as to an optical disc such as DVD or the like, a light source unit 161 such as a laser diode or the like including an emission unit for emitting an optical beam with a wavelength of 405 nm as to a high-density recording optical disc, a common objective lens 164 for DVD and CD, and a diffraction optical device 165 for aberration correction.
Also, this optical pickup 160 includes a movable collimator lens 167, ¼ wavelength plate 175, redirecting mirror 176, beam splitters 168A, 168B, and 169A, gratings 173A, 173B, and 173C, and so forth.
Further, the optical pickup 160 includes a multi lens 172B and detector 171B as a return light detection system for optical disc such as DVD, CD, or the like, a multi lens 172A and detector 171A as a return light detection system for high-density recording optical disc, and a beam splitter 169B for introducing a predetermined optical beam to each of the return light detection systems, within an return optical path.
An optical beam with a wavelength of 785 nm or so emitted by the light source 163 is reflected at the beam splitter 168B, transmits the beam splitter 169A, and inputs to the objective lens 164. The optical beam is condensed on a signal recording face of an optical disc having a protection layer with thickness of 1.1 mm or so by the objective lens 164.
An optical beam with a wavelength of 655 nm emitted from the light source unit 162 is reflected at the beam splitter 168A, transmits the beam splitters 168B and 169A, and inputs to the objective lens 164. The optical beam is condensed on a signal recording face of an optical disc having a protection layer with thickness of 0.6 mm by the objective lens 164. The return light with a wavelength of 785 nm and the return light with a wavelength of 655 nm which are reflected at the signal recording faces of the optical discs are reflected at the beam splitter 169B through the beam splitter 169A, and are detected by the detector 171B including a photodetector or the like through the multi lens 172B.
An optical beam with a wavelength of 405 nm or so emitted from the light source unit 161 is transmitted through the beam splitters 168A, 168B, and 169A, and inputs to the objective lens 164. The optical beam is condensed on a signal recording face of an optical disc having a protection layer with thickness of around 0.1 mm by the objective lens 164. The return light with a wavelength of 405 nm reflected at the signal recording face of the optical disc is transmitted through the beam splitter 169B through the beam splitter 169A, and is detected by the detector 171A including a photodetector and so forth through the multi lens 172A.
With the return system of the optical pickup 160, the focal lengths and locations of the multi lens 172B for DVD/CD and the multi lens 172A for high-density recording optical disc are adjusted as appropriate, whereby the return magnifications of the optical beams with a wavelength of 785 nm and wavelength of 655 nm for optical disc such as DVD, CD, and so forth, and the return magnification of the optical beam with a wavelength 405 nm for high-density recording optical disc can be set, and a focus capture range suitable for each of the wavelengths can be provided.
With the optical pickup 160 such as shown in FIG. 13, the objective lens 164 compatible with three-wavelength optical beams, and diffraction optical device 165 for aberration correction are provided, thereby realizing recording and/or playback of different three types of optical discs, and also setting a focus capture range suitable for each of the optical discs, i.e., realizing compatibility between multiple types of optical discs.
However, with the optical pickup 160 shown in FIG. 13, the number of components is increased by providing two detectors including a light receiving element, thereby causing a problem from the perspective of reduction in costs, and reduction in size. That is to say, with such an optical disc 160, for example, employing the two detectors causes increase in costs, and also a device such as a beam splitter for separating and inputting optical beams corresponding to these detectors is used, also, multiple components such as a multi lens serving as a magnification transformation lens for condensing each optical beam on the light receiving element of each detector are used, and wiring is laid from two places, which causes the configuration to become complex, thereby leading a problem such that the configuration becomes complex as a whole, and also reduction in size of the device is prevented, or the like.
Thus, with the above-mentioned optical pickups shown in FIGS. 12 and 13, there has been a problem wherein the number of components increases, and the optical systems become complex. Also, providing multiple objective lenses or light receiving elements cause problems such as increase in processes for adjusting these, taking a great amount of time for manufacturing an optical pickup, the configuration of the device becoming complex, and impeding reduction in size.
Thus, it has been very difficult to satisfy both of that the return magnification corresponding to each format is optimized from the perspective of setting the focus capture range to a suitable range, and that a common objective lens and common light receiving element is employed as to each use wavelength corresponding to each optical disc, thereby enabling three-wavelength compatibility, reduction in size of the device, and simplification of the configuration thereof.